1. The Field of the Invention
This invention relates to telecommunications systems. More particularly, the present invention relates to systems and methods for echo cancellation during high speed data communications over telephone lines.
2. The Prior Art
Modern telecommunication systems allow almost any two points in the world to be placed in voice or data communications with each other. While the telephone system which is in place is generally efficient for its intended purpose, it inherently includes several undesirable characteristics. One such undesirable characteristic is the echo present in full duplex telephone systems.
Echo in a telephone system is often unavoidable. In the case of a telephone call between two local points, the local connection from the phone to the local telephone exchange is made by a two wire loop. In the case of a telephone call between two distant points, the connection (e.g., microwave or satellite link) will be converted from the bidirectional two wire loop to two unidirectional channels equivalent to two pairs of wires.
The device which accomplishes the two to four wire conversion is generally referred to as a "hybrid." In theory, all of the signal from the incoming channel of the four wire circuit is coupled to outgoing channel of the four wire circuit. Unfortunately, in practice, a significant amount of the energy is not coupled from the incoming signal to the outgoing signal. The remaining energy is conveyed back to the originator where it is perceived as an echo.
In short distance communications, the echo often involves delays of less than 40 milliseconds and a human user will not perceive a discrete echo. Still, even echoes which are delayed less than 40 milliseconds can disrupt high speed data communications. If a satellite link is in the communication channel, the echo may be delayed for over a second causing serve disruption of data and voice communications. Even though the level of the echo is generally down 11 dB from the original signal level, it is disruptive if not corrected, especially when data is being transmitted via a modem.
Echoes present on telephone channels are generally classified as "near-end" echoes and "far-end" echoes. Near-end echoes are strong and are generally caused by imperfections found in the equipment located at the local telephone switching office. Since the source of the near-end echo is nearby, there is usually little time between the transmission of the original signal and the receipt of the echo back at the transmitting device. Far-end echoes are generally weaker than near-end echoes and are defined as those echoes caused by imperfections in the telephone channel between the distant telephone switching office and the distant user. Since the source of the far-end echo is distant, the time between the transmission of the original signal and the receipt of the echo back at the transmitting device often is one-half of a second or more.
One approach which has been used to cancel echoes involves the use of a device often referred to as a "vari-losser" or a "switched loss device." Such an approach dynamically changes its connection to the telephone channel depending upon whether the near-end talker is speaking or the far-end talker is speaking. When the far-end talker is speaking, the path used to transmit the near-end speech is opened so that echo is prevented. Conversely, when the near-end talker is speaking, the same path is closed and a similar arrangement on the far-end is opened.
While the use of a vari-losser device provides adequate echo cancellation when the telephone channel is carrying human speech, even though in actual practice it's drawbacks include clipping speech sounds and impairing speech interruptions, it severely hinders high speed data communications.
Another principal approach which has been used in the past to construct an echo canceller consists of a filter in the form of a tapped delay line. The coefficients at each tap of the delay line define the transfer characteristic of the filter. For the tapped delay line to work as an echo canceller, the transfer characteristic found at one of the taps must match that of the echo path between the modem transmitter and modem receiver.
The "delay line filter" is used by a modem to estimate the echo return from the waveform to be transmitted by processing that waveform through the filter. Then, as the waveform is transmitted, the estimated echo is subtracted from the received signal.
Importantly, there have been numerous attempts to improve the methods whereby the tap coefficients are made adaptive so that the correct tap values can be learned during "training periods" when the echo canceller determines the correct filter characteristics which match the echo so that it can be subtracted out of the received signal. Other similar methods allow the tap coefficients to track any slow changes in the echo path as they occur during a communication session. Techniques which use tapped delay line methods are limited by processing speed to approximately 60-70 taps and thus the accuracy with which they are able to match the echo is limited.